1. Technical Field
The present disclosure relates generally to building automation system (BAS) control networks, and more particularly, to BAS devices which dynamically allocate termination resistors in BAS communications networks.
2. Background of Related Art
A building automation system (BAS) is used to coordinate, manage, and automate control of diverse environmental, physical, and electrical building devices and systems, particularly heating, ventilation, and air conditioning (HVAC) systems. Other building systems such as lighting, security, and environment management systems may also be controlled and monitored by a BAS. A BAS includes a BAS controller that can use hardwired and/or wireless communications networks using open and/or proprietary protocols to link various endpoint BAS devices, such as variable air volume (VAV) boxes, chiller plants, rooftop units, variable refrigerant flow (VRF) systems, blowers, convectors, user interface devices, and/or sensors, to provide system-wide user access, monitoring, and control. Many wired BAS installations communicate using the TIA/EIA RS-485 standard to implement the controller area network (CANbus) network protocol introduced by Robert Bosch GmbH of Gerlingen, Germany.
RS-485 provides a differential balanced line twisted pair physical layer. Typical RS-485 network devices used in BAS installations employ a termination resistor to impedance-match the signal lines at the network connection. Termination of RS-485 connections is required to minimize electrical reflections in the network which would otherwise degrade network performance. Typically, an installer will set a dual inline package (DIP) switch located on a printed circuit board (PCB) of the BAS device to enable or disable the termination resistor during the initial system configuration process. In other instances, header pin jumpers may be used to select termination resistor status, traces cut or resoldered, or a termination resistor may be physically inserted or removed from the PCB as needed. However, there is no defined procedure that installers and technicians follow to determine when or where to enable or disable the termination resistor. Usually a combination of experience, luck, and trial & error will determine which termination resistors are needed in a BAS installation. In large and complex environments, this is a tedious and time-consuming task that can lead to a “good enough” solution that falls short of achieving optimal network performance.
In addition, modern PCB designs now use miniaturized surface mount devices (SMD) to save space and reduce the cost of PCB fabrication. Since a DIP switch is a relatively large device compared to an SMD, designs which employ DIP switches require more PCB space, have an expanded form factor, and cost more to produce. Another drawback of DIP switches in a BAS device is that they can be easily misconfigured, and can experience electrical or mechanical failures that degrade network performance in a manner that can be extremely difficult to troubleshoot.